how to generate error message in form if any of the fields left blank

how to generate error message in form if any of the fields left blank

<form onsubmit="chkform()">
<table>
<tr><td>name</td><td><input type="text" id="uname"/></td></tr>
<tr><td></td><td><div id="er1"></div></td></tr>
<tr><td>address</td><td><input type="text" id="add"/></td></tr>
<tr><td></td><td><div id="er2"></div></td></tr>
</table>
</form>
<script>
function chkform()
{
if (document.getElementById("uname").value === "" )
{
document.getElementById("er1").innerHTML = "name cant be
left blank";
document.getElementById("er1").style.color = "red";
document.getElementById("er1").style.display = "block";
}
}
</script>
i want to show error message in div, if any of the fields are left blank
and this message should disappear when text box is clicked.